The present invention relates to a screw conveyor for transporting powdered materials such as, for example, powdered foods, powdered chemicals, powdered pharmaceuticals, and the like.
Materials in powder form include granulated sugar, glucose, powdered skim milk, starch, rice bran, seasonings, cement, fly ash, carbon black, salt, detergent, wheat, rice, maize, soybean, malt, quartz sand, aluminum oxide, titanium oxide, calcium carbonate, dust coal, lime, soda ash, powder food mixes, flour, powder soup, powder petrochemicals, pharmaceuticals, pigments, mixed fertilizer, phenol resins, polyethylene resins, ABC resins, PVC powder, polypropylene powder, powder coating, and the like. These are supplied as powder raw material to the manufacturing company. After being processed in the manufacturing company, these powder materials are generally packed in bags or plastic containers and supplied to the market.
In small- and medium-scale factories, the powder material, typically shipped by trucks or freight cars, is used straight from the plastic containers and bags. In large-scale factories, the usage amount is much greater, and the shipping and handling of the material involves heavy labor. In order to improve the work environment technology for automated conveyance of powder is in practice.
The technology for automated transport of powder material involves first bringing in the material into a storage tank from a truck or rail car by air conveyance. Air conveyance uses a blower for air conveying the material to a holding tank. A screw conveyor runs through each production line for sending the powder material to temporary storage tanks of a processing apparatus. But for the technique described above, the screw conveyor transports material by rotating a helical blade of the screw conveyor inside a tube casing.
In a conventional screw conveyor, a gap between the edge of the rotating blade and the interior wall of the casing permits some of the powder material to adhere, and thus to and remain in this gap as the material is being transported. This is a special problem if the powder material is food matter, or a spoilable material, since long retention of a residue in the screw conveyor may permit the material to spoil or to permit insect infestation. Spoiled or insect-infested material in the screw conveyor may become dislodged, thereby permitting this undersirable material mix with a powder currently being transported. Not only does this degrade quality, but it also produces unsanitary conditions.
An additional problem arises from the powder accumulating in one direction on the inside wall of the casing. This can displace the screw toward the opposite wall to reduce the clearance between the blade edge and the casing sufficiently, in a worst case scenario, to force the edge of the screw conveyor blade into the casing wall on the opposite side from the buildup. This can cause wear, damage and stoppage of operation.
Upon the occurrence of such problems, the usual practice is to slop the conveyor and to wash the conveyor with water. But, in order to wash with water, everything attached to the screw conveyor must be removed. This is costly in terms of labor and downtime, and is thus a major inconvenience. After washing the casing, its entire interior must be dried completely before it can be returned to operation. Only then may the processing machinery be restarted. As a result, productivity is lower, and the financial losses that result can be a critical problem for the whole plant.